The superior high temperature and erosion resistant properties of rigid pyrolytic graphite materials are well known. These properties make the material particularly useful as liners for chambers or vessels subject to such conditions, as rocket nozzle inserts, and the like.
Pyrolytic graphite, however, does have certain disadvantageous properties stemming from its particular crystallite structure and from its tendency to oxidize, particularly at high temperatures in an oxidizing atmosphere.
Pyrolytic graphite is normally produced by the pyrolysis of a carbonaceous gas, such as methane or propane, onto a heated substrate. Flat, hexagonal crystallites oriented parallel to the substrate surface are deposited in layers which build up into an essentially laminar structure. The pyrolytic graphite crystal is considerably wider in its flat or a-b plane than along its thickness dimension or c-axis. As a result, pyrolytic graphite is highly anisotropic in many of its properties, including strength, heat conductivity and thermal expansion, with attendant difficulties in practical use. As an example, the material has an exceedingly high coefficient of thermal expansion in the thickness or c-axis direction and a relatively low coefficient in the a-b direction. As a result, it is exceedingly difficult to match a pyrolytic graphite liner or insert with a suitable backing material which can avoid separation during thermal cycling. Because of its weakness in the c-direction, due to its flat, plate-like and, thereby, laminar microstructure, pyrolytic graphite tends to delaminate under high stresses.
The embedding within the laminar pyrolytic graphite crystallite structure of aciculae of crystalline SiC which are oriented in the c-direction, as compared to the planar orientation of the layers of the pyrolytic graphite in the a-b direction, advantageously reduces the anisotropy of the graphite and reduces the tendency of the graphite to delaminate. Additionally, it substantially improves oxidation resistance since, unlike carbon which oxidizes to a gas, silicon oxidizes to SiO.sub.2 which fuses to form a protective coating. Improved oxidation-resistance is particularly important if the pyrolytic graphite is exposed to high temperature oxidative atmospheres.
The production of SiC films and coatings, for example, on flexible metal filaments such as tungsten, by vapor phase pyrolysis of a silane, such as SiH.sub.4, SiCl.sub.4, SiHCl.sub.3, (CH.sub.3).sub.4 Si or CH.sub.3 SiCl.sub.3 with or without added hydrocarbon gas, is well known, the objective generally being the production of pure SiC. The pyrolysis temperatures employed are generally below the optimum temperatures for producing pyrolytic graphite.
Seishi Yajima et al, Journal of Materials Science 4 (1969) pp. 416-423 and 424-431, and Chemical Abstracts, 1970, 7, p. 69, disclose a structure comprising flake-like single crystals of SiC dispersed in a matrix of pyrolytic graphite and oriented parallel to the planes of the graphite. The crystallite size of the SiC was about 200 A thick (c-direction) and about 2000 A in diameter (a-b direction). Since the single SiC crystals of the Yajima et al structures are essentially flat and oriented in the same planar direction as the pyrolytic graphite, they cannot have any substantial effect on the anisotropy or delamination characteristics of the latter.
Yajima et al pyrolyzed a mixture of SiCl.sub.4 and propane under vacuum. Maximum SiC production of up to 4 weight percent was obtained at temperatures of about 1400.degree.C to 1500.degree.C and dropped to as little as 0.02 to 0.03 weight percent at temperatures of about 2000.degree.C. Since SiC is considerably denser than pyrolytic graphite, the volume percent of SiC was substantially smaller.
None of the referenced art discloses the pyrolytic graphite-SiC microcomposite of this invention or the process for making it.
Copending applications Ser. No. 592,846 now U.S. Pat. No. 3,629,049, dated Dec. 21, 1971 and 870,948 now U.S. Pat. No. 3,715,253, dated Feb. 6, 1973 disclose rigid pyrolytic graphite articles comprising a matrix of pyrolytic graphite containing embedded therein at least one reinforcing layer consisting of a plurality of unidirectional and substantially parallel, laterally spaced, individual, continuous carbon strands. The matrix comprises crystallite layers of pyrolytic graphite nucleated from each of the individual carbon strands and interconnected to form a continuous phase surrounding and interconnecting the individual strands comprising the embedded strand layers. By conforming the crystallite pyrolytic graphite layers to embedded strand surfaces instead of to the surface of a conventional base substrate, anisotropy of the pyrolytic graphite and its attendant disadvantages are substantially reduced.
Utilization of the codeposited pyrolytic graphite-SiC microcomposite of the present invention in place of the pyrolytic graphite matrix disclosed in said copending applications provides further improvement in isotropy and improves oxidation resistance.
The object of the invention is to provide a rigid pyrolytic graphite-SiC microcomposite having substantially lower anisotropy than pyrolytic graphite and improved oxidation resistance.
Still another object is to provide a process for making said rigid pyrolytic graphite-SiC microcomposite.
Another object is to provide rigid reinforced composite pyrolytic graphite-SiC articles having additionally decreased anisotropy.
Still another object is to provide a process for making said rigid reinforced composite pyrolytic graphite-SiC articles.
Other objects and advantages will become apparent from the following description and drawings.